The present invention relates to an interconnection structure of semiconductor devices and, more particularly, to a metal interconnection which has a barrier metal layer formed thereunder and which is also used to form electrode lead-out portions for external connection.
With the reduction in the size of semiconductor integrate circuits, the depth of PN junctions has decreased. As a result, it has become common practice to employ Al-Si alloy as an interconnection metal in place of Al. The Al-Si alloy has the advantage that it is possible to form an ohmic contact with a shallow N-type diffused layer without destroying the junction.
The Al-Si alloy suffers, however, from the disadvantage that, as the contact area decreases, Si which is contained in excess of the limit in solid solution undesirably precipitates into the contact portion in the form of P-type Si doped with Al during H.sub.2 annealing process, causing an increase in the effective contact resistance. The circumstances of the phenomenon will be explained below more specifically.
For example, in a 2 .mu.m-rule semiconductor integrated circuit, the diameter of a contact hole provided in an insulating film to realize contact between a diffused layer and a gate electrode on the one hand and a metal interconnection on the other, that is, the contact area, is relatively large. Accordingly, even if silicon precipitates into the contact portion of the metal interconnection which is connected to the diffused layer through the contact hole, the precipitation of silicon will not spread over the whole contact portion and therefore the contact resistance will not rise markedly. However, it has been confirmed that in the case of a 2 .mu.-rule semiconductor integrated circuit wherein the diffused layer which is connected with a metal interconnection is an N.sup.+ -diffused layer which is formed by diffusion of phosphorus, the contact resistance is increased by the precipitation of silicon and a contact failure is likely to occur. This is because, when an N.sup.+ -diffused layer is formed by diffusion of phosphorus, crystal defects are unlikely to occur in the surface of the semiconductor substrate in comparison with other cases (e.g., the case where an N.sup.+ -diffused layer is formed by ion implantation of arsenic) and therefore silicon is likely to grow in the form of a single crystal.
On the other hand, in the case of a 1.3 .mu.m-rule semiconductor integrated circuit, the diameter of a contact hole formed in an insulating film, that is, the contact area, is relatively small. Therefore, the precipitation of silicon readily spreads over the whole contact portion of the metal interconnection, so that the contact resistance is markedly increased, which often results in a contact failure. As the size of semiconductor integrated circuits shrinks more and more, the above-described problem will become critical.
In order to prevent occurrence of a contact failure due to the precipitation of silicon, it is conventional practice to dispose a barrier metal layer of a silicide of a metal, for example, Mo, underneath a metal interconnection which is connected to a diffused layer. More specifically, the presence of the barrier metal layer suppresses the alloying reaction of Al with Si, so that it is possible to realize an ohmic contact with the diffused layer. This technique is described, for example, in "Ultra-High Speed MOS Devices", pp. 95-96, published from K. K. Baifukan on Nov. 15, 1986.
According to a typical conventional method, such a barrier metal layer has heretofore been formed as follows. After a through-hole has been formed in an interlayer insulating film on which a metal interconnection is to be formed, Mo silicide is formed the through-hole and on the whole surface of the interlayer insulating film by sputtering. Thereafter, Al (containing Si) is formed on the whole surface of the Mo silicide by sputtering and then the Al and Mo silicide layers are patterned. Techniques concerning the above-described barrier metal layer are disclosed, for example, in Japanese Patent Laid-Open Nos. 62-89343 (1987), 62-43175 (1987) and 62-43176 (1987).